We love our car (2005 on a 54 plate C-Max 1.6 TDCi Manual Zetec) but we have had an unresolved engine management issue with it since the middle of 2012. The engine management warning light came on (on Friday 13 July!) and immediately the car lost all power with a system failure message. The car was eventually taken to our local Ford dealer when our usual garage was unable to even start the car. Ford were initially unable to diagnose the problem as their diagnostic equipment kept on throwing up several fault codes including instrument cluster failure. The pcm and wiring cluster was checked cleaned and car reassembled and all seemed OK and nothing was replaced. This cost us £489.41 in labour. The car then started misbehaving a month later (losing power, speedo falling to zero and recovering, engine warning light on) and then they replaced the accelerator pedal sensor (FOC) after much complaining. Now at the end of the year, the problem has reappeared, and this happened after I drove in torrential rain and through several slightly flooded sections of road. The warning messages we are getting include, 'Limited Speed Mode, 'Steering Assist Failure', the Speedo fails occasionally and the car will lose power momentarily when we are decelerating which is really scary, but then pick up again. Ford have in all fairness tried to sort the problem out so no real complaints there. Does have anyone have any suggestions?

I would suspect a Speedo sensor.
It's possible that you have damaged wiring to the sensor.
Water ingress to a connection or exposed wire can cause this problem.
I'm not sure if your model of car has the sensor on the gearbox or on the hubs but this may well be worth investigating.

Update:
Car went into the Ford Dealer on Wed 2 January. Their diagnostics pointed to a faulty instrument cluster. This was replaced on Friday 4 at a cost of £700!. Saturday 5, the Engine Management warning light came on again having driven the car for 4 miles. The car has been back in the garage since Monday 7. We have got Ford UK involved but neither they nor the garage are able to track the real problem down. I am wondering if we are going to get our £700 back. Incidentally, we have since found out that there is a company - ecutesting.com who can test and repair instrument clusters and ecu / pcm units at a fraction of the price.

Hi, I have a 56 plate focus 1.6 tdci, it has just had the same issue but it happened 3 times in 1 mile, after turning the car off a few times it drove normally again. Luckily I was near my local garage so they are going to look at it in the morning. At the time it was pouring with rain and has been all day so my thinking is some water is somewhere it shouldn't be.